This invention relates in general to methods of producing polyimide foam products and, more specifically, to the production of a foam having improved physical characteristics.
A number of methods of manufacturing polyimide foam products have been developed, such as those described by Lavin in U.S. Pat. No. 3,554,939, by Gagliani et al in U.S. Pat. Nos. 4,394,464 and 4,407,980 and by Long et al in U.S. Pat. No. 4,476,254.
Lavin et al in U.S. Pat. No. 3,483,144 describes a process for preparation of a polyimide foam which is the reaction product of a tetracarboxylic acid or tetracarboxylic acid dianhydride and a polyamine containing from 2 to 6 amino groups per molecule. The preferred tetracarboxylic acid is 3,3,4',4'-benzophenone tetracarboxylic acid. A polyimide foam on the basis of 4,4'-oxydiphthalic acid or anhydride is not disclosed.
Other prior methods of making polyimide foam such as disclosed in U.S. Pat. No. 3,554,939 generally combined a single monomeric polycarboxylic acid ester component, such as 3,3', 4,4' benzophenone tetracarboxylic acid dianhydride (BTDA) converted to the half-ester, with one or more monomeric polyamine components. Others reflux a dianhydride such as BTDA with an oxoimine or lactame such as caprolactam, until esterification is complete, then add the diamine or diamines. The resulting precursor is dried and can be caused to spontaneously foam by heating to an appropriate temperature.
While these prior techniques often produce foam products having excellent characteristics for particular purposes, we have found they do not have the specific tensile strength and flexibility and low density desired for many applications. Thus, there is a continuing need for improved polyimide foams and methods of making them.
It is a further object of this invention to provide a process for the preparation of polyimide foams having improved flexibility, low density and specific tensile strength.
It is a further object of this invention to provide such polyimide foams.